The invention relates to a device for generating a laser beam in a LADAR system for use in a target tracking missile.
A LADAR operates similar to a RADAR. Instead of microwaves used in a RADAR, LADAR uses laser radiation at shorter wavelengths such as infrared or visible light. Accordingly, LADAR permits higher resolution both with respect to distance and with respect to angle than RADAR. Thereby, a profile of a target can be scanned. Such profile measurement, better recognition of the type of target may be possible. Furthermore information about the velocity and the direction of the movement of the target can be obtained from the LADAR signals. Higher pulse rates of pulses emitted by the laser result in higher scanning rates laser and shorter wavelength results in higher resolution.
LADAR systems are used for target seeking and target tracking. The requirements for a LADAR-transmitter used for target seeking and target tracking are very sophisticated. The used laser must have a high pulse frequency and a high pulse energy. The laser parameters, such as amplitude, frequency, phase, polarization and the duration of the emitted pulse must be capable of modulation in order to generate different pulse shapes.
Known laser assemblies for LADAR are based on discrete lasers with a laser resonator comprising mirrors, which have to be adjusted with high precision. The used arrangements are therefore large, heavy and sensitive to vibrations and temperature changes. Furthermore the known laser assemblies require cooling means in order to achieve sufficient pulse energy, the cooling means generally comprising water cooling assemblies. Thereby the assembly will be even larger. With many laser assemblies, the formation of large temperature gradients also prevents high pulse repetition rates to be achieved. However, such high pulse repetition rates and, consequently, high scanning rates are a requirement, if high relative velocities are to be measured.
For these reasons, LADAR, in the current practice, has been used on ground only. The prior art assemblies are not suited for the use in missiles.
Fiber optic lasers are known from telecommunications applications. A fiber optic laser comprises a fiber with a laser-active core and a light guiding periphery: Light from a pump light source is guided into the laser active core, which results in laser activity. Such a fiber optic laser does not require additional water cooling, because it has a high surface-to-volume ratio. An example for such a fiber optic laser is an erbium doted glass fiber.
In U.S. Pat. No. 5,847,816 to Zediker et al. a micro-doppler ladar system for identifying and analyzing a target is described, which makes use of a fiber optic power amplifier for amplification of the radiation from a master oscillator. The ground based LADAR system operates with a coherent transmitter-receiver-assembly and a fiber optic master oscillator.
The use of a fiber optic power amplifier as a preamplifier in the detection path of a Laser-doppler-anemomometer is described in the paper xe2x80x9c40 dB fiber optical preamplifier in 1064 nm Doppler anemometerxe2x80x9d by T6bben, Buschmann, Muller and Dopheide, Electronics Letters 36, p. 1024. The described fiber optics power amplifier amplifies the power of a Nd:YAG-laser, which is a powerful laser by itself.
Furthermore microchip lasers are known. A microchip laser comprises a laser active medium in the form of a very thin plate, whereby a short resonator length is achieved. An example for a microchip laser is described in the data sheet of Leti, CEA/Grenoble, xe2x80x9c1,5 xcexcm passively Q-switched Microchip Lasersxe2x80x9d in September 2000. The peak power of such a microchip laser is according to this publication 1-4 kW with pulse widths of about 3 ns. Repetition rates of 1 to 20 kHz can be achieved and the output power is in the order of 30 10-65 mW.
Hellstr6m, Karlsson, Pasiskevicius and Laurell disclose the use of a microchip laser with an additional amplifier on the conference xe2x80x9cAdvanced Solid-State Lasers 2001xe2x80x9d on 28.-31.1 2001 with the title xe2x80x9cAn optical parametric amplifier based on periodically poled KTi 0 P04, seeded by an Erxe2x80x94Yb:glass microchip laserxe2x80x9d. The amplifier used in this case, however, does not operate with a fiber optic power amplifier.
None of the above described lasers is adapted for use in a LADAR system which is suitable for missiles.
It is an object of the invention, to provide a lightweight device for generating a laser beam in a LADAR system.
It is a further object of the invention to provide a device for generating a laser beam, which is insensitive with respect to vibrations and temperature changes.
It is a still further object of the invention to provide a device for generating a laser beam, which is particularly compact and which is therefore suitable for the use in a missile.
It is a further object of the invention to provide a LADAR system with a laser which generates pulses with high energy and a high repetition rate without the need of water cooling.
To achieve these objects, a carrier body of a material with high heat conductivity is provided, on which said fiber optic power amplifier is coiled. With such a carrier a compact arrangement is obtained which is particularly suitable considering the limited space available in seeker heads of missiles. If the carrier body consists of a material with a high heat conductivity the heat losses of the laser can be dissipated through the carrier body. Heat conducting paste, in which the fiber optic power amplifier is embedded, can be used to increase the dissipation of excess heat. The fiber optic power amplifier can also be affixed to the carrier by means of heat conducting glue.
A two-stage arrangement provides the advantage that a laser with comparatively short wavelength and low output power can be used and its light can then be amplified to the required output power. By moving the generation of a high output power to a fiber optic power amplifier a complicated cooling system can be avoided. The remaining heat generated by the laser can be dissipated through the carrier body. As the minimal pulse width of a laser is proportional to the resonator length, very short pulse widths can be realized with accordingly short resonator lengths. A two-stage arrangement with a master oscillator with short resonator length and low output power and a fiber optic power amplifier connected to its output provides short pulses with a high repetition rate and high pulse energy. For modulating the emitted laser pulses only the low-energy master oscillator needs to be modulated.
Therefore the laser assembly is a very simple arrangement and can be realized with small 15 diameters. The master oscillator can be exchanged without changing the power amplifier. Such a laser assembly is suitable for missiles. It has high heat stability and is insensitive to acceleration and vibrations.
The use of a fiber optic power amplifier permits the generation of laser radiation in the 20 eyesafe spectral range with wavelengths which are larger than 1,5 microns, which means that the radiation will not be focused by the human eye.
Preferably the master oscillator is a microchip laser. A microchip laser consists of a thin laser active plate, for example a 1 mm thick erbium: glass plate. These microchip lasers are capable of generating radiation with short wavelengths and have only very small outer diameters. As the radiation power is amplified afterwards their comparatively small radiation power is irrelevant.
Furthermore a diode laser can serve as a pump light source for the fiber optic power amplifier. Diode lasers are small, cheap and easy to handle and they are, therefore, particularly suitable for the use in missiles.
A recess may be provided in the carrier body, in which the diode laser can be arranged. Thereby the assembly will be even more compact. The carrier body may have a further recess in which the pump light source and/or further optical components can be arranged. Preferably the recess can be closed with a cover. Thereby the components arranged in the S recess are protected from damaging influences.
Preferably the carrier body is cylindrical or is prismatic with an elliptical or reniform cross section, a recess being provided for the components in its end faces and the fiber optic is coiled around the circumference. Such a geometry is particularly suitable for the 10 use in missiles, as it contains very small unused spaces only. By developing the geometry of the fiber optic the absorption efficiency in the fiber can be optimized. However, the bending radii of the fibers must not drop below a permissible minimum. Flat surfaces on various sides of the cylinder are also possible.
In a further embodiment of the invention an optical Faraday insulator is provided between the master oscillator and the fiber optic power amplifier. Such an insulator acts like an optical diode. It prevents the backscattering or reflecting of radiation from the amplifier into the master oscillator. Thereby any unwanted laser activity is avoided.
The coupling of the pump radiation into the fiber optic power amplifier preferably is achieved through a dichroitic beam splitter in front of the fiber optic power amplifier. The pump light can be coupled into both ends of the fiber optic power amplifier. A coupling is also possible by means of a fiber optic. In a further modification the fiber optic power amplifier is a double core fiber and the fibers of the pump laser are directly connected with the pump core. Thereby an assembly is provided which is particularly insensitive to vibrations and very compact.
A fiber may be provided for transporting the laser radiation to the transmitter optical arrangement of the LADAR system. In this case the laser assembly can be arranged outside of the seeker head of the missile, where more space is available.
An embodiment of the invention is described below in greater detail with reference to the accompanying drawings.